#define _CRT_SECURE_NO_WARNINGS 1
/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int ret = -1, count = 0;
    int kthSmallest(TreeNode* root, int k) {
        if (count == k) return ret;
        if (root == nullptr) return ret;

        kthSmallest(root->left, k);
        if (count < k) count++;
        if (k == count && ret == -1)
        {
            ret = root->val;
            return ret;
        }
        kthSmallest(root->right, k--);
        return ret;
    }
};